This invention generally relates to solid oxide fuel cells, and is particularly concerned with a stress reducing mounting between an electrolyte sheet assembly and a support frame in such a fuel cell.
Solid oxide fuel cells incorporating flexible electrolyte sheet assemblies are known in the prior art. In such fuel cells, one or more electrolyte sheet assemblies are supported within a housing between a pair of mounting assemblies, which might be either a frame or a manifold.
In a solid oxide fuel cell device assembly that incorporates a multiple cell design, such as that disclosed in U.S. Pat. No. 6,623,881 assigned to Corning Incorporated, the electrolyte sheet assembly may include an electrolyte formed by a ceramic sheet of zirconia doped with yttrium oxide (Y2O3) that is between 18-20 microns thick. The doped zirconia sheet supports a plurality of rectangular cells, each of which is formed by an anode and cathode layer on either side of the doped zirconia sheet, and each of which may be between 4-8 microns in thickness. A current collector layer overlies the cathode and anode layer, each of which may be about 20 microns thick and formed from a composite of a silver/palladium alloy and yttria stabilized zirconia (YSZ). The resulting sheet assembly is only about 60 microns in thickness in its central portion, where the cells are arrayed, and has a border region formed solely by the supporting doped zirconia electrolyte sheet, which is only between 18-20 microns in thickness. Such a thin structure advantageously affords a flexibility to such an electrolyte sheet assembly which allows it to withstand the thermal shock associated with many cycles of heating from ambient temperature to a range of 700° C.
In a single cell design, the electrolyte sheet assembly is supported by a ceramic anode layer, which is between 100-1000 microns in thickness and formed from a composite of nickel and yttria stabilized zirconia. Such single cell electrolyte sheet assemblies further include a thin electrolyte layer overlying the anode layer, and a cathode layer overlying the electrolyte. Unlike the multiple cell design, the border portion of single cell sheet assemblies has the same thickness as the central portions, as the structure of the single cell is generally not confined to the central portion of the sheet. Additionally, the single cell design is not as flexible as the previously described multiple cell design due to its greater thickness and lower strength due to the porosity. However, despite their greater stiffness and lower strength, such single cell electrolyte sheet assemblies have proven to have sufficient thermal cycling tolerance to render them practical.
Unfortunately, the stresses and strains imposed upon both types of electrolyte sheet assemblies from thermal shock gradients and thermal expansion due to the many cycles of heating and cooling cause such sheets to fracture over time, which ultimately degrades the power output produced by the solid oxide fuel cell. To solve this problem, it has been suggested in the prior art that a pattern of corrugations be incorporated into the ceramic layers forming the electrolyte sheet to reduce stress and strain. See in particular U.S. Pat. No. 5,519,191 assigned to Corning Incorporated. While such corrugations have proven to be effective, they can cause fabrication difficulties with some types of low cost cell/multi-cell manufacture.
Clearly, what is needed is a way to eliminate, or at least reduce, the amount of fracturing that occurs in flexible electrolyte sheets as a result of the stresses and strains imposed upon them from thermal expansion. Ideally, such a solution should be compatible with both multi-cell and single-cell electrolyte sheet assemblies, and should be easy and inexpensive to manufacture within the sheet assemblies or other components of the solid oxide fuel cell.